1. Field of the Invention
The present invention is directed toward a sound generating device, which may find particular application in a combination back-up light and sound generating system for use in an automobile, in which the sound generated is made more audible.
The present invention is further directed toward a circuit which can drive a transducer, such as a piezoelectric transducer, and which can drive the transducer at its resonant frequency even when a transformer is placed between the driving circuit and the transducer.
2. Discussion of the Background
Disclosed in U.S. Pat. No. 4,851,813, which is herein incorporated by reference, is a combination back-up light and sound generating device for an automobile. This device operates so that when the automobile is placed in reverse, the sound generating device will generate a sound to provide an audio indication that the vehicle is being backed up. According to this device, this sound generating device is located in the same housing as the back-up light. This device is shown and described with reference to FIGS. 1 and 2. As shown in FIGS. 1 and 2, the combination back-up light and sound generating device includes a housing 20 which can be of any shape, but is hexagonal in section in the illustrated embodiment. The housing 20 has a base 22 which is similar to the base of a standard bulb to be inserted into the tail light socket of the automobile, and also includes bayonet type projections 24 corresponding to the bayonet type projections of the standard bulb.
At the end of the housing opposite the base, the housing supports a conductive socket 26 which is electrically connected to socket 22 via wires 30a and 30b to provide electrical continuity between the tail light socket 10 and the socket 26.
An electrically operated sound generating device 32 is positioned within the housing 20. This sound generating device 32 is a piezoelectric transducer consisting of a piezoelectric ceramic material. The sound generating device operates to vibrate the surrounding air and thereby generate sound. Openings 34 are formed on top of the housing to permit the generated sound to be emitted therefrom.
This device, however, suffers from the drawback that in practice the sound pressure level outside of the tail light assembly is low, even if a loud sound is generated by the transducer. The tail light assembly lens cover creates a sealed housing which damps or muffles the generated sound. One possible solution to this problem is to use a larger piezoelectric transducer. However, this solution is not feasible since there are size constraints as to the maximum possible size of the piezoelectric transducer. Another possible solution is to increase the sound generating efficiency of a given sized transducer. If one wants to create very loud sounds using piezoelectric technology, one must drive the piezoelectric element with high voltages and at its resonant frequency. For example, 50 to 250 volts peak to peak are common for such products as the combination backup-light and sound generating device or car alarm sirens.
Each piezoelectric transducer to be utilized for such a function will have a different resonant frequency F.sub.O which is a function of its size, construction, unit to unit differences, the temperature at which it operates and other factors. To achieve the loudest sound possible at an output of a piezoelectric transducer, it is important to drive the piezoelectric transducer at its resonant frequency and to drive it with high voltages. However, each piezoelectric transducer from the same manufacturing batch differs slightly and thus has a different resonant frequency. Therefore, to drive each piezoelectric element at its resonant frequency F.sub.O to achieve the loudest possible sound, it would be necessary to "tune" each unit at the factory for the particular piezoelectric transducer enclosed therewith. This would greatly add to the cost of producing such units. Also, even if a unit is so "tuned", it may still not operate over a wide temperature variation, as the resonant frequency F.sub.O drifts due to temperature changes, thereby causing a drastic reduction in sound level outputs.
One method of solving this problem is to track the resonance of the piezoelectric transducer in real-time and to incorporate a feedback circuit to ensure that the piezoelectric transducer operates at its resonant frequency F.sub.O under all conditions. This approach will typically include the piezoelectric transducer in the signal path of an oscillator. The signal passing through the piezoelectric transducer will be greatest at its resonant frequency F.sub.O and, therefore, the oscillator should operate at that frequency. A known feedback circuit used to track the resonant frequency of a piezoelectric transducer to achieve its loudest output is shown in FIG. 6.
As shown in FIG. 6, a piezoelectric element 10 is connected at one side to ground and at the other side to two terminals P1 and P2. Terminal P1 is a main terminal which receives a driving signal to drive piezoelectric element 10. Terminal P1 is connected between the piezoelectric element 10 and a node Z1. Connected between this node Z1 and a node X1 is a capacitor C1. Also, node X1 is connected to the second terminal P2. Connected between node Z1 and a further node Y1 is an inverter amplifier A2. Located between nodes X1 and Y1 is a resistor R1. Also, located between nodes X1 and Y1, in parallel with resistor R1, is a series combination of a resistor R2 and a second inverter amplifier A1. In this way, the output of inverter amplifier A1 is used as the input into inverter amplifier A2.
The device of FIG. 6 operates in the following manner. The output of inverter amplifier A2 is attached to main terminal Pl of piezoelectric element 10. The signal for driving piezoelectric element 10 is applied to terminal Pl to thereby drive the piezoelectric element 10. As a result, the piezoelectric element 10 distorts and vibrates due to the driving signal applied thereto. A voltage produced by this distortion is then sensed by terminal P2 which is also attached to piezoelectric element 10. The voltage sensed at terminal P2 is then applied to inverter amplifier A1 through resistor R2 and is then inverted and amplified again by inverter amplifier A2 to produce an amplified replica of the original signal detected at terminal P2. The magnitude of amplification provided by amplifying inverters Al and A2 should be chosen so as to ensure that the final signal applied to driving terminal P1 will generate oscillation in the piezoelectric element 10.
The frequency of oscillation detected by terminal P2 will be the resonant frequency of the piezoelectric element 10, as this is the frequency at which the signal passes most easily from driving terminal P1 to terminal P2. Thus, terminal P2 will sense the resonant frequency of the piezoelectric element 10 and will apply that signal, after it is amplified, to driving terminal P1 to ensure that the piezoelectric element 10 continues to oscillate at its resonant frequency. In this way, the piezoelectric element 10 will operate at the frequency at which it can output a maximum sound. The resistor R1 is provided to ensure a proper operating point and duty factor of the driving circuit. Resistor R2 protects amplifying inverter A1 from receiving an over voltage at its input. Capacitor C1 provides hysteresis, making the oscillator more efficient.
This system described with reference to FIG. 6, however, has certain drawbacks.
The amount of power transferable from the driving circuit to the mechanical vibration of the piezoelectric element 10 (this power ultimately being converted into sound) is proportional to the square of the peak-to-peak output voltage of the driving circuit and is inversely proportional to the effective resistance of the element at its resonant frequency F.sub.O. If the maximum available output voltage is limited to a DC supply voltage, it may be impossible to get the full output from the driving circuit and piezoelectric element transducer assembly.
Further, when the piezoelectric element is required to be driven at very high voltages, for example, 50 to 200 volts peak-to-peak, the only way to achieve this type of voltage level from a standard 5 to 18 volt power supply circuit is to use a step-up transformer. However, the circuit shown in FIG. 6 cannot operate effectively with a step-up transformer. The problem is that the step-up transformer introduces phase shifts that result in a tendency of the complete circuit to oscillate at frequencies other than the preferred resonant frequency F.sub.O. Therefore, if a transformer is used in the circuit shown in FIG. 6, terminal P2 will not sense a signal at the resonant frequency F.sub.O through the piezoelectric element and, therefore, the driving signal applied to terminal P1 will also not be at the resonant frequency F.sub.O of the piezoelectric element. Thus, the piezoelectric element will not resonate at its resonant frequency F.sub.O and therefore the piezoelectric element will not operate to produce the loudest sound possible.